System for timing exposure interval of photographic prints



Oct. 11, 1949. A 2,484,299

SYSTEM FOR TIMING EXPOSURE INTERVAL v OF PHOTOGRAPHIC- PRINTS Filed OCT,- 20, 1947 IN VEN TOR. N 7. LnBRl/M ATTORNEY Patented Oct. 11, 1949 SYSTEM FOR TIMING EXPOSURE INTERVAL OF PHOTOGRAPHIG PRINTS Ervin D. Labrum, Seattle, Wash.

Application October 20, 1947, Serial No. 780,908

7 Claims.

This invention relates to the art of printing from photographic negatives, and has for its p ticular object to 'providean electronic timer which will automatically control the time interval in which a print is exposedto light rays emanating from the exposurel amp.

With this general object in mind and which will appear and be understood in the following description and claims, the invention consists in the arrangement, adaptation, and combination of pants hereinafter described and claimed.

In the accompanying drawings:

Figure 1 is a wiring diagram illustrating the preferred embodiment of a severalty of electric elements associated in a manner operative toaccomplish the end object of the present invention.

Fig. 2 is a schematic elevational view representing the relationship which the photo-cell of the present invention bears to the negative, the print, and the exposure lamp of'a printing lay-out when the printing is being performed by what is-commonly known as the contact procedure; and

Fig. 3 is a view similar to Fig. 2 but showing the relationship of these parts when a projection procedure is resorted to in performing the printing operation.

Clarity in an understanding of the present invention will perhaps be advanced by firstnpointing' out that the present device employs a photoelectric vacuum tube sensitive to rays of light, subjecting this tube only tothe light being used to expose the print, and utilizing the photo-cell to operate a relay which in turn control-s an electric circuit including the exposure lamp. In the instance of cont-act printing, and as will be seen from an inspection of Fig. 2, the photo-cell, denoted by the numeral 5, is placed such that it is affected by the light rays emanating fromv an exposure lamp 6 only after the intensity of these rays is modified by their passage first through the negative 7', then the printingpapers, and finally through a diffusing screen 9'. While these several parts I, 8 and 9 are, for better perception, indicated as being separated one-from the other, normal contact printing, as the name implies, would find the negative, the-paper, and the screen in touching or closely proximate relation. When exposure is accomplished by the projectionprocedure illustrated in Fig. 3, and in which the light from the lamp 6. is: projected through the negative 1 onto the printing paper '8, I then desirably place the photo-cell above and to one side of the printing paper and rely upon reflection from the sensitized surface of the paper to acti- 2 vate the 'cell, the placement of the screen S-being functionally the same as with contact Printing, and namely in a position between the paper and the photo-cell for intercepting and difiusing the rays of light.

Now to consider the circuit, and which derives its electric current through main leads Ill-and I I while applying its current to the lamp through leads l2 and I3, there is employed, with the lamp and with the photo-cell 5, a severaltyof vacuum tubes. While two will sufiice, it is desirable that three be employed and these I denote 14, i5 and I6. Tubes suitable for the purpose are, respectively, 6SJ7, 605, and 2050. Tied in with the lamp, the cell and these tubes is a relay H, a transformer 8, a rectifier tube 2t! for the transformer, and certain switches, condensers, resistore, and potentiometers, all of which will appear from an inspection of the drawing and whose functions, where not self-evident, will be hereinafter pointed out, largely in course of tracing the operation. Using the letters S, C, R and P, with difierentiating exponents, to respectively denote these switches, condensers, resistors and potentiometers, the operation may be described as follows:

Let it be assumed that the system is in the condition in which it is illustrated in Fig. 1, and namely with switch 8-! closed and switches S-2 and S3 open. These switches are manually operated and the arrangement is desirably one in which the two switches Sl and S2 are ganged together to operate in unison, the opening of the one simultaneously closing the other. To now prepare the system for operation, a circult is caused to be completed through the power lines l0l I by closing a master switch (not shown). The resulting positive voltage on potentiome'ter P affects the condenser C to charge the latter. By such charging, a positive voltage is placed on the plate and screen of the tube It but by reason of the grid of the tube being biased to the cut-off no current passes the tube. The potentiometer P and the resistor R are on the negative side of the power supply and the directive flow of the current is such that point y is negative with respect to the ground. The potentiometer P has as its function a control of the circuit such that the grid of the tube It is minus to the cut-01f point, being at cut-off when the photo-cell. 5 is unafiected by any light. The fact that the plate element of the tube I4 is positive and, has the grid of tube 1 5 direct-coupled thereto perforce makes the grid of the latter tube positive and this responsively causes current to flow through the load resistor R There thus develops a voltage drop across the resistor, and the provision of a direct connection from the positive side of the resistor to the cathode of the tube l6 makes the control grid of this tube negative with respect to the cathode. Having a sharp cutoff characteristic of 2 volts negative in the instance of a 2050 gas triode, no plate current flows through the relay I1 and the contact points of the latter are hence closed, but it will be understood that the circuit through lead lines l2|3 to the exposure lamp 6 is broken in that the switch S is open.

Now, to energize the lamp for initiating an exposure operation, the operator actuates the ganged switches S and S to open the former and close the latter, whereupon the circuit to the lamp is completed. As the light rays responsively reach the photo-cell 5 the condenser C commences to discharge, this bein permitted due to the fact of the current flow being more positive at point z of the potentiometer P to offset the bias cut-off conditions of the tube M. The greater the intensity of the light striking the photo-cell, the faster the rate of discharge from the condenser C and as this discharge proceeds to such an extent as causes the control grid of tube 15 to reach the negative bias cut-off point, plate current fiow through resistor R ceases and, with no voltage drop, the control grid of tube [6 becomes zero with respect to the cathode and the tube I6 passes its maximum current, responsively breaking the contact of the relay I] to deenergize the exposure lamp 6. This will have completed a printing cycle and the procedure is repeated for successive printing operations, first activating the ganged switches to again simultaneously close switch S and open switch S to effectuate a charging of the condenser C and then closing switch 8 while simultaneously opening switch S Having an understanding of the operating nature of the parts, it will be self-evident that minor departures may be resorted to without sacrifice of the inventive concept, in which respect it is particularly pointed out that the tube [5 might be deleted. When so deleted, and with direct connection between tubes l4 and Hi, the latter would be constantly drawing plate current while the exposure lamp is on and the resulting system would be somewhat less sensitive.

The purpose of the various electrical parts which I have elected to illustrate and not hereinbefore expressly mentioned will be largely apparent but may be briefly set forth. P and R act in complement as a single variable resistor, and their ofiice within the system is that of an adjusting means for either increasing or decreasing the amount of load resistance transmitted to the tube I4, causing greater or less voltage drop with any given current flow. This variable re-. sistor is effective when light rays of minor intensity play upon the photo-cell, namely when using a dark or thick negative, and there is ordinarily no need for any change in the setting once the same has been set by initial tests made upon the system. This is likewise true of the setting given the potentiometer P which lies in the cathode circuit of the tube l4 and is experimentally set to properly dampen out brighter light to which the photo-cell is subjected or, more objectively stated, modifying the time interval of exposure for proper handling of light or thin negatives. P is in the bias circuit of the tube to control the cut-off point.

The remaining potentiometer, and namely P is the only one of the variable resistance elements which calls for any manual adjustment after the system has been test-set, and this unit is manually controlled in compensation of differences in the emulsion speeds of printing which exist between various grades of paper.

P R and R are hooked in series to produce a voltage divider assurin a voltage drop across R and P of the magnitude required to obtain the proper voltage upon the cathode of the tube l5.

The resistor R and the condenser C are essentially a part of the power circuit and perform a filtering office to obtain a required voltage drop on the photo-cell, and in this respect it may be stated that an efiicient working of the system is or may be obtained by impressing volts on the power supply and 20 volts on the cathode of the tube l5, this latter voltage being necessarily lower than the voltage upon the cathode of the tube 16. With the figures given the voltage drop across the resistor R should be in the neighborhood of 15 volts.

The two resistors R and R are each a highohm unit and are employed to protect the circuit, acting to affect the ratio of discharge in case a'tube, in consequence of being defective, draws grid current. The faulty tube then evidences itself and can be replaced before damage results to the system. The two leads marked XX serve merely as a pilot-light connection evidencing a live circuit through the main leads l0 and H.

The salient teaching of the present invention is the concept of using a photo-cell, a relay, 2. tube for controlling the relay, and a condenser serving as a reservoir, with the operational procedure, for timing the exposure interval, being one in which recourse is had to the time necessary to accomplish a draining of the reservoir charge. The system is shown as incorporating a means, and namely the switch S the closing of which makes the system non-automatic and allows manual control of the exposure time should the latter, for any reason, be desired.

The present system gives to the operator a means of automatically and accurately controlling, to a fractional part of a second, the exposure of a print, the conventional procedure having been previously one in which the operator resorts to an inspection of the exposed negative to estimate the proper time interval and then either closes and opens a circuit to the exposure lamp manually or by the employment of a timer which he first sets to the estimated time. For commercial printing, the present invention not only appreciably speeds up production with improved printing, but also reduces waste to a minimum and enables inexperienced operators to be employed with assurance of satisfactory returns from the automatic system, it being necessary simply that the operator activate a control member common to the two switches S and S moving the same in one direction to charge the condenser and in an opposite direction to open the circuit to the lamp which responsively excites the photocell and initiates a dissipation of the condensers charge.

The invention, the manner of its operation, and its accomplishments are believed to be clearly understood from the foregoing. While having illustrated and described the embodiment of the invention now preferred by me, minor departures therefrom may obviously be resorted to without 2,484,299 g g 6 d parti eir'om e sp of t i e io a d i controlled by the tube, and includes the condenser is myintention that the hereto annexed claims be and the relay together withelectrical devices conread with .a scope commensurate with the nected in shunt with the condenser and operating broadest interpretation h c e e pl y d to hold the relay inactive when the condenser is guage permits. 5 in a charged condition and to activate the relay What I claim is: upon a dissipation of the charge, the electrical de- 1. The described electronic system for .autovices last-named being operative by a coincident matically timing the exposure interval when opening of the last-named and a closing of the printing from an exposed negative and which first-named of the two manually operated switchcomprises, in combination: an exposure lamp, a 10 es to efiectuate a discharge of current from the photo-electric tube energized by rays of light condenser at a predetermined rate correspondin emanating from the lamp and passing through to the intensity of the light rays striking the tube.

the negative, and a severalty of electric circuits 5. The described electronic system for automatone of which includes the lamp, another of which ically timing the exposure interval when printing includes the tube, and still another of which from an exposed negative and which comprises, in includes a condenser acting to normally carry a combination: an exposure lamp, a photo-electric storage charge of current, a vacuum tube and a tube energized by rays of light emanating from series resistor provided in the condenser circuit the lamp and passing through the negative, a recontrolled by an energizing of the tube connected lay, and a severalty of electric circuits one of which in shunt with the condenser and acting tocause includes the lamp and two circuit-closing switches the condenser to be bled of its stored charge and in series, one of these switches being normally to responsively open the lamp circuit when disclosed and opened by activation of the relay and sipation is complete, said electrical means acting the other being manually operated, another of to effectuate a discharge of current from the which circuits includes a manually operated circondenser at a predetermined rate corresponding 5 cuit-closing switch and a condenser acting to reto the intensity of the light rays striking said ceive and store a charge of current when the cirtube cuit is closed, and still another of which circuits 2. The system of claim 1 having a manually is inter-related with the second-named circuit, is operated switch for closing the last-named circontrolled by the tube, and includes the condenser cuit, and a second manually operated switch ingo and the relay together with electrical devices controduced in the lamp circuit and arranged to nected in shunt with the condenser and operating work in complement with the electrical devices to hold the relay inactive when the condenser is such that the circuit through the lamp is closed in a charged condition and to activate the relay only by its combination action therewith, the upon a dissipation of the charge, the electrical two switches being adapted to work oppositely in 3 evices last-named being operative by a coinciunison the one being open while the other is dent opening of the last-named and a closing of n s d, the first-named of the two manually-operated 3. The described electronic system for autoswitches to effectuate a discharge of current from matically timing t exposure t m when the condenser at a predetermined rate correprinting from an exposed negative and which 44 p n to intensity of 1lhe light y tri n comprises, in combination; an exposure lamp, the tube, the tWO manually-operated switches bea photo-electric cell energized by rays of light ing ganged together in a manner to W k pp emanating from the lamp and passing through sitely in unison with the one being open while the the negative, a relay, an electric circuit including other is C osed. the lamp and two circuit-closing switches in 45 6. In an electronic system o a ca y series one of which is manually operated and timing the exposure interval When printing fro the other normally closed and opened by activaan exposed negative: t e Combination of an extion of the relay, a second electric circuit includ- P051116 lamp, 3 Photo-electric tube energized y ing the relay and a condenser arranged to carry rays of light passing t'lhlollgh the negative, the a storage charge of current and also including, quantum of light reaching the light-sensitive eleconnected in shunt with the condenser, electric ment of the tube directly effecting the u t of devices which operate to hold the relay inactive current flowing through Said element, n electric h t condenser i in a charged condition control-circuit containing said tube and including while activating the relay upon a dissipation of a resistor, Condenser in Said Circuit normally the charge and which are controlled by the photocarrying a Storage Charge of urr nt, a vacuum 11 such as t responsively cause the condenser tube and a series resistor controlled by an enerto lose its charge by an energizing of the cell. and gizing 0f the Photweleetrie tube d C n ected in then t a predetermined t corresponding to shunt with the condenser in such a manner that t intensity of t light rays striking the cell when bias voltage of the vacuum tube is effected 4. The described electronic system for automatby energizing y n e light-sensitive element i n timing t exposure interval when printing the condenser will be bled of its charge and the from an exposed negative and which comprises, lamp circuit responsively opened as the lowering in combination; an exposure lamp, a photmelem charge potential of the condenser reaches a predetric tube energized by rays of light emanating termined level. from t lamp and passing through the negative, 7. The described electronic machine for autoa relay, and a severalty of electric circuits one of matically timing the exposure nt val when which includes the lamp and two circuit-closing printing from an eXpOSed negative in the p ocess switches in series, one of these switches being of Contact p ting, the photo reco d and the sennormally closed and pened by activation of t sitized material being in close proximity to each relay and the other being manually operated, another, and which comprises, in combination, an other of which circuits includes a manually operexposure p, a photo-electric e, and means ated circuit-closing switch and a condenser acting for olding said ne tive and sensitized material to receive and store a charge of current when the in close proximity and in a printing position, the circuit is closed, and still another of which circuits components of the machine being so arranged as is inter-related with the second-named circuit, is to have light rays from the printing lamp pass through both the photo record being printed and the sensitized material being exposed before reaching the photo tube.

ERVIN D. LABRUM.

REFERENCES CITED The following references are of record in the file of this patent:

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